Photomechanical polymers are a special class of stimuli-responsive materials that can transduce light into mechanical work. Photo-initiated shape adaptivity or force generation (actuation) are particularly intriguing due to the salient features of light, namely, remote and wireless (contactless) triggering with ease of spatial, temporal, directional (through polarization), and magnitude (with intensity) control.
Considerable effort has been undertaken in both the synthesis of photoresponsive polymers and the characterization of their photomechanical outputs. To date, azobenzene has been the most-utilized photochromic unit in the examination of amorphous, crystalline, and liquid crystalline polymers because of its excellent thermal stability, resolved isomeric forms, unique optical nonlinearities, and ability to form surface reliefs when subjected to conventional or polarization holography. The resulting photomechanical output of a polymeric material is dependent not only on its optical properties (absorption wavelength, wavelength of exposure, polarization of exposure) but also on its morphology (amorphous, crystalline, liquid crystalline) and thermomechanical properties.
A number of reports have distinguished the photo-responses of liquid crystalline polymer networks (LCN, both glasses and elastomers) for comparatively large magnitude responses typified by bending of cantilevers or dramatic uniaxial contractions of thin films. Notably, a majority of these efforts have characterized the response of azobenzene-based LCN to exposure to UV light, which is known to decrease the order of the LCN through trans-cis photoisomerization and can result in an isothermal phase transition. UV-induced responses in azobenzene LCN are limited in the need for multiple light sources to reverse the trans-cis isomerization.
Comparatively, we have explored the use of 442 nm (or 488 nm) exposure of azobenzene LCN materials and demonstrated distinct photomechanical responses such as polarization controlled forward and reverse bending of a cantilever, as reported in U.S. patent application Ser. No. 13/272,775 filed on Oct. 13, 2011, which is incorporated by reference herein in its entirety. Recently, the photomechanical and thermomechanical response of a glassy polydomain azobenzene LCN has been documented, revealing the close relationship between these two properties.
Heat-resistant polymers such as polyimides (PIs), poly(amide-imides) and polyamides are useful in a variety of applications. Particularly, the wide application of PIs is a result of their excellent combination of physical properties, thermal stability, and processability. For example, PIs containing azobenzene in the backbone or side-chain have been investigated for photo-induced alignment in liquid crystal display (LCD) as well as nonlinear optical applications. More recently, an azobenzene-containing poly(amic acid) (a PI precursor) was crosslinked by a triamine in N,N-dimethylformamide (DMF) and the resulting sol-gels showed a two-fold increase in the storage modulus after irradiation with 405 nm light. And we recently developed a new photomechanical polymer system derived from a multi(azobenzene-amine) cross-linker, an aromatic amine and a dianhydride, as disclosed in U.S. Pat. No. 8,785,589 and U.S. patent application Publication Ser. No. 13/866,524, each of which is incorporated herein by reference in its entirety. The resultant crosslinked azobenzene-containing glassy polyimides possessed favorable photomechanical and thermomechanical responses.
Accordingly, there is a need for new azobenzene-containing monomers, as well as new polymers made therefrom, for new materials having photomechanical and thermomechanical responses.